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Land Subsidence: Cause & Effect

Land subsidence—the loss of surface elevation due to removal of subsurface support—occurs in nearly every state in the United States. Subsidence is one of the most diverse forms of ground failure, ranging from small or local collapses to broad regional lowering of the earth's surface. The causes (mostly due to human activities) of subsidence are as diverse as the forms of failure, and include dewatering (oxidation) of peat or organic soils, dissolution in limestone aquifers, first-time wetting of moisture-deficient low-density soils (hydrocompaction), natural compaction, liquefaction, crustal deformation, subterranean mining, and withdrawal of fluids (groundwater, petroleum, geothermal).

The compaction of susceptible aquifer systems caused by excessive groundwater pumping is the single largest cause of subsidence in California, and the 5,200 mi2 affected by subsidence in the San Joaquin Valley since the latter half of the 20th century has been identified as the single largest human alteration of the Earth's surface topography. The second largest cause of subsidence in California is the oxidation (decomposition) of organic soils.

Aquifer Compaction due to Groundwater Pumping


Background

Although land subsidence caused by groundwater pumping has caused many negative effects on human civil works for centuries, especially in the highly developed urban or industrialized areas of Europe, the relation between subsidence and groundwater pumpage was not understood or recognized for a long time. Recognition began in 1928 when pioneer researcher O.E. Meinzer of the U.S. Geological Survey realized that aquifers were compressible. At about that same time, Karl Terzaghi, working at Harvard University, developed the one-dimensional-consolidation theory. The theory states generally that compression of soils results from the slow release of pore water from stressed clay materials and the gradual transfer of stress from the pore water to the granular structure of the clay.

Cause

Fine-grained sediments (clays and silts) within an aquifer system are the main culprits in land subsidence due to groundwater pumping. Fine-grained sediments are special because they are composed of platy grains (imagine the shape of dinner plates). When fine-grained sediments are originally deposited, they tend to be deposited in random orientations (imagine haphazardly placing your dinner plates in the sink). These randomly oriented sediment grains have a lot of room between them to store water. However, when groundwater levels decline to historically low levels, those randomly oriented sediments are rearranged into stacks (imagine plates stacked in the cupboard). These stacks occupy less space and also have less space between them to store water.

Diagram illustrating the effects long-term groundwater pumping has on an aquifer.

Effect

The effects of compaction fall into two categories: those on manmade infrastructures and those on natural systems. The greatest effects occur to infrastructures that traverse a subsiding area. In the San Joaquin Valley, the main problems reported are related to water conveyance structures. Many water conveyance structures, including long stretches of the California Aqueduct, are gravity driven through the use of very small gradients; even minor changes in these gradients can cause reductions in designed flow capacity. Managers of the canals, such as the California Department of Water Resources, the San Luis Delta-Mendota Authority, the Bureau of Reclamation, and the Central California Irrigation District, have to repeatedly retrofit their canals to keep the water flowing...albeit at reduced amounts. While water conveyance structures tend to be the most sensitive to subsidence, damage to roads, railways, bridges, pipelines, buildings, and wells also can occur.

While more focus has been placed on the highly visible infrastructure damage from subsidence, which generally can be repaired, compaction of the aquifer system, sight unseen, may permanently decrease its capacity to store water; subsidence occurring today is a legacy for all tomorrows. Even if water levels rose, compacted sediments would remain as-is; most compaction that occurs as a result of historically low groundwater levels is irreversible. Additionally, as the topography of the land changes by varying amounts in different places, the low areas, such as wetlands, will change size and shape, migrate to lower elevations, or even disappear. Rivers may change course or erosion/deposition patterns to reach a new equilibrium.

Decomposition of Organic Soils


Background

The Sacramento-San Joaquin Delta of California was once a great tidal freshwater marsh. It is blanketed by peat and peaty alluvium deposited where streams originating in the Sierra Nevada, Coast Ranges, and South Cascade Range enter San Francisco Bay. In the late 1800s, levees were built along the stream channels, and the land thus protected from flooding was drained, cleared, and planted ('reclaimed'). The leveed tracts and islands help to protect water-export facilities in the southern Delta from saltwater intrusion by displacing water and maintaining favorable freshwater gradients. However, ongoing subsidence behind the levees, where the land has been drained, exposed to the atmosphere, and planted, increases stresses on the levee system, making it less stable, and thus threatens to damage agricultural and developed lands and degrade water quality in the massive north-to-south water-transfer system.

Cause

The dominant cause of land subsidence in the Delta is decomposition of organic carbon in the peat soils. Under natural waterlogged conditions, the soil was anaerobic (oxygen-poor), and organic carbon accumulated faster than it could decompose. Drainage of peat soils for agriculture led to aerobic (oxygen-rich) conditions. Under aerobic conditions, microbial activity oxidizes the carbon in the peat soil quite rapidly. Most of the carbon loss from the soil occurs as a flux of carbon-dioxide gas to the atmosphere.


Diagram illustrating how peat loss from reclamation results in land subsidence.

Effect

Reclamation and agriculture have led to subsidence of the land surface on the developed islands in the central and western Delta at long-term average rates of 1 to 3 inches per year (Rojstaczer and others, 1991; Rojstaczer and Deverel, 1993). Islands that were originally near sea level are now well below sea level, and large areas of many islands are now more than 15 feet below sea level. The landsurface profile of many islands is somewhat saucer-shaped, because subsidence is greater in the thick peat soils near their interior than in the more mineral-rich soils near their perimeter. As subsidence progresses, the levees themselves must be regularly maintained and periodically raised and strengthened to support the increasing stresses on the levees that result when the islands subside. Currently, they are maintained to a standard cross section at a height 1 foot above the estimated 100-year-flood elevation of the adjacent channels. Water levels in the depressed islands are maintained 3 to 6 feet below the land surface by an extensive network of drainage ditches, and the accumulated agricultural drainage is pumped through or over the levees into stream channels. Without this drainage, the islands would become waterlogged.